To decorate gypsum wallboard, until recently the only manner in which this could be accomplished was by a process separate and apart from the actual formation of the board from the slurry of calcined gypsum. That is, the tendency of the paper cover sheets to "blow" have prevented a coating of paint, which conventionally is relatively impermeable, from being applied to a board wherein the core is set but still wet, prior to kiln drying. In the past, any impermeable substance applied to the paper cover sheet prior to drying the core either caused the sheet to separate or delaminate from the core as moisture is driven off in the drier, or required impractically slow drying speeds. As will be obvious, the more impermeable the coating composition became, such as by the addition of cellulosic thickeners to paint, the more a sheet coated with the same was blown off the core during drying.
Thus, decorations such as paint have either been applied previously to the cover sheet, which tends to damage the decoration as the boards are cast with the predecorated face resting on the support, or they have been applied to the completely dry and otherwise finished board. Either procedure is relatively expensive as it involves a separate handling and drying step. It has long been desired to produce a decorated board wherein the drying of the decorations, as well as of the core, can be accomplished in a single drying step.
Recently, a process was provided as disclosed in U.S. Pat. No. 3,507,684, which accomplished the goal of a single drying step. The painting or other coating of the still-wet board was rendered possible in that process by adding solids to the coating which rendered the cover sheet, not the coating itself, more porous than it would otherwise be. Such solids were pressed into the cover sheet, thus penetrating the same so that moisture could escape when driven off during the drying. The resulting board of necessity had a textured decorative appearance, and the examples disclosed in said patent were limited to a paint wherein the binder was a large particle size homopolymer emulsion. There is disclosed only the use of acrylic emulsions as thickeners, cellulosic thickeners having been specifically disclaimed.